1. Field of the Invention
The present invention relates to a cell searching method, more particularly, to a method for quickly searching a cell at a mobile station by using a synchronization channel structure of a forward link in a code division multiple access (CDMA) system which operates by an asynchronous mode between base stations.
2. Prior Art
In an interim standard (IS)-95 CDMA system, all base stations operate by using a frame synchronization between base stations based on an external time such as a global positioning system (GPS) receiver. In a wideband CDMA system which has been proposed in Europe or Japan as an international mobile telecommunication system-2000 wireless transmission technique, all the mobile stations can operate without a frame synchronization between base stations differently from that of the IS-95 CDMA system. In the synchronous IS-95 CDMA system, a mobile station discriminates each base station based on a phase difference with respect to an absolute time of one code sequence. On the other hand, in an asynchronous wideband CDMA system which has been proposed in Europe or Japan, the mobile station discriminates each base station based on different code sequences. Since the asynchronous wideband CDMA system does not need the GPS receiver, a cell searching time of a mobile station is lengthened at an initial calling set or a hand off. In order to decrease a cell searching time in the asynchronous wideband CDMA system, it uses two synchronization channels in a forward link.
U.S. Pat. No. 5,765,111(issued to Kyou-Woong Kim on Jun. 9, 1998) discloses a method and apparatus for selectively calling a mobile station by generating a ring signal at only a selected mobile station among a plurality of mobile stations registered to a private base station.
FIG. 1 shows a synchronization channel structure of a forward link which is used for a conventionally synchronous wideband CDMA system. The synchronization channel (SCH) of a forward link includes a primary SCH(C) and a secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16). The primary SCH(C) is composed of a binary code having 256 chips which are not modulated. Each code of the primary SCH(C) is transmitted every slot at a slot boundary. The slot has a length of 0.625 msec. One frame of the primary SCH(C) has a time interval of 10 msec. As mentioned above, each code of the primary SCH(C) is transmitted every slot, that is, sixteen codes of the primary SCH(C) are repeatedly transmitted in every frame. In the synchronous wideband CDMA system, all base stations use an identical code as the primary SCH(C). The primary SCH(C) is used for searching a starting location of 16 slots per one frame.
The secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16) is composed of sixteen code sequences of an orthogonally binary gold code which have not been modulated. Each of the sixteen code sequences corresponds to each of the primary SCH(C). Each element of the secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16) is selected from an orthogonally binary code group (C1, C2, . . . , C16, C17) each having seventeen elements. That is, Citxcex5{C1, C2, . . . , C16, C17}. The secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16) indicates a code group (group i) to which a long code belongs. A current base station uses the long code. 512 different long codes are used in the system and divided into 32 long code groups. Each of the 32 long code groups is discriminated based on the secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16). That is, 32 different secondary SCHs are present at the system. The secondary SCH uses a hopping code such as a Reed-Solomon code. An alphabet size and a length of the secondary SCH are 17 and 16, respectively. 32 possible sequences of the secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16) are unique with respect to a cyclic shift. For example, an optionally cyclic shift, a code sequence length of which is smaller than 16 and is not zero, is always different from a cyclic shift except the optional cyclic shift, a code sequence length of which is smaller than 16. Such a feature of the secondary SCH is used for uniquely determining a long code group and a starting point of 10 msec frame by means of a mobile station.
An initial synchronization obtaining method by a mobile station will be described referring to FIGS. 1 and 2. FIG. 2 illustrates an initial synchronization obtaining method by a mobile station.
In step S201, a mobile station (not shown) searches a predetermined starting point of a slot with respect to a base station (not shown) having a minimal channel loss by using a primary SCH(C) as shown in FIG. 1, by means of a matched filter (not shown).
In step S202, the mobile station obtains long code group information and frame sync information of 10 msec. The long code group information and frame sync information of 10 msec are obtained by correlating 17 possible secondary SCH(Ci1, Ci2, . . . , Ci15, Ci16) with respect to a mobile station receiving signal at a slot location. The step S202 will be described in detail below. The mobile station calculates decision variables with respect to 512 sequences which includes 32 possible sequences and 16 cyclic shifts each that the 32 possible sequences may have. The decision variables are obtained by non-coherently adding 17 correlator outputs with respect to a sequence corresponding to a corresponding decision variable at each of 16 secondary synchronization channel locations. The mobile station obtains the long code group information and frame sync information by selecting a maximum value among 512 decision variables.
In step S203, the mobile station judges what kind of code is the long code based on the long code group information and frame sync information, that is, a frame location information (frame boundary) obtained in step S202. That is, correlating operations with respect to 16 long codes included in the long code group obtained in step S202 are performed to obtain correlating results. When a maximum value of the correlating results is larger than a set threshold value, the mobile station judges that the long code having the maximum value is a code for a band spreading of a forward link which a current base station uses.
However, the conventional cell searching method is satisfied with orthogonal feature between a forward synchronization channel and a traffic channel. An interference between a synchronization channel and a traffic channel with respect to a signal received through an identical path of an identical cell occurs. The interference causes reduction in a capacity of a forward link and increases a searching time of a mobile station. The conventional cell searching method needs two sync channels and a mobile station needs 17 parallel correlators so that it has a complex configuration. Since the 17 parallel correlators should be used to continue searching a neighboring cell for a handoff after a call set, power consumption is increased.
Therefore, it is an object of the present invention, for the purpose of solving the above mentioned problems, to provide a cell searching method which provides long code group information and frame sync information by using a location of a sync channel code with respect to a starting point of a slot by one sync channel.
In order to attain the object, according to the present invention, there is provided a cell searching method in an asynchronous wideband code division multiple access system, said method comprising the steps of:
(a) obtaining a predetermined starting point among a plurality of locations of sync channel codes in a frame with respect to a base station having a minimal channel loss;
(b) obtaining long code group and frame sync information at the mobile station based on outputs of a matched filter every mini slot clock location for one frame time after a starting point of a current mini slot; and
(c) detecting a long code based on the long code group and frame sync information obtained in step (b) wherein the long code is used for band-spreading of a forward link by a current base station.
Preferably, step (b) includes (b-1) calculating a plurality of decision variables corresponding to each of a plurality of pseudo time hopping code sequences based on the outputs of a matched filter; (b-2) selecting a pseudo time hopping code sequence corresponding to a decision variable having a maximum value among the plurality of decision variables; and (b-3) obtaining the long code group and frame sync information based on the pseudo time hopping code sequence selected in step (b-2).
There is also provided a cell searching method in an asynchronous wideband code division multiple access system, said method comprising the steps of:
(i) selecting one maximum value per slot, that is, L (where, L is an integer greater than one as the number of slots per frame) pieces of maximum value per frame based on outputs of a matched filter with respect to a sync channel code and selecting S(where, 2xe2x89xa6Sxe2x89xa6L) pieces of values from the L pieces of maximum values;
(ii) calculating (NGROUPxc3x97Lxc3x97S) decision variables corresponding to each of a plurality of pseudo time hopping code sequences with respect to each location which corresponds to the S pieces of selected maximum values, the NGROUP represents the number of long code groups which are used in the system;
(iii) obtaining long code group and frame sync information based on the calculated (NGROUPxc3x97Lxc3x97S) decision variables; and
(iv) detecting a long code based on the long code group and frame sync information obtained in step (iii) wherein the long code is used for band-spreading of a forward link by a current base station.
Furthermore, there is provided a cell searching method in an asynchronous wideband code division multiple access system, said method comprising the steps of:
selecting K values (where, L and K are an integer greater than 1) per frame among a plurality of sampling values every one slot at L pieces of slots, that is (Lxc3x97K) maximum values per frame;
calculating (NNEIGHBORDxc3x97Lxc3x97Kxc3x97L) decision variables corresponding to each of a plurality of pseudo time hopping code sequences with respect to each location which corresponds to the (Kxc3x97L) selected maximum values wherein the NNEIGHBORD represents the number of groups of a neighboring cell a mobile station should search in a mobile station idle or active state;
obtaining long code group and frame sync information of a neighboring cell based on the calculated (NNEIGHBORDxc3x97Lxc3x97Kxc3x97L) decision variables; and
detecting a long code that is used for band spreading of a forward link by a current base station based on the detected long code group and frame sync information.